A. Hyper-V in Windows Server 2008 R2 supports up to four virtual processors per virtual machine (VM), but different guest OSs running within those VMs are tested with different numbers of processors—not all guest OSs are supported for use with the full four virtual processors. The full list of OSs and supported number of virtual processors can be found on this Microsoft site.

If you install a guest OS that, for example, is listed as only supporting two virtual processors, but you configure the VM with four virtual processors, it will still boot—there's no hard limit. However, the configuration wouldn't be supported, and there might be valid performance reasons why more virtual processors aren't supported, so you might see poor performance. But there's nothing that would actually stop you.